1. Field of the Invention
This invention relates to a method and apparatus to aid in the maintenance and repair of the lower internal assembly of a nuclear reactor vessel, and more specifically, to a method and apparatus which provides a frame capable of inverting the lower internal assembly so that repair and maintenance operations may be performed with greater speed, thus reducing the exposure of workers to radiation.
2. Background Information
Because of the radiation hazard present while performing repair and maintenance operations on the components of a nuclear reactor, it is desirable to limit the exposure of workers to any radioactive components during such maintenance and repair procedures. A nuclear reactor includes a stationary reactor vessel which encloses a removable reactor core assembly. The reactor core assembly includes two main assemblies, the upper internal assembly and the lower internal assembly. For example, the upper internal assembly includes the control rod drive mechanisms, control rod drive shafts and the upper core plate. The lower internal assembly includes the core barrel, lower instrumentation guide tubes, tie plates and a baffle. The nuclear fuel assemblies or fuel cells are maintained within the core barrel between the upper core plate and the lower core plate.
It is known in the prior art to perform maintenance operation on the lower internal assembly. Access to the lower internal assembly is gained by removing the reactor vessel upper head assembly and the upper internal assembly, including the upper core plate. This procedure exposes the fuel assemblies which are also removed. Once the upper internal assembly, which includes the upper core plate, and fuel assemblies have been removed, the lower internal assembly may be removed from the reactor vessel. When removed from the reactor vessel, the lower internal assembly is set on a storage stand which supports the lower internal assembly above the maintenance bay floor. While on the maintenance stand the lower end of the core barrel is approximately thirty feet under water.
Typically, the only procedure performed on the lower internal assembly is the inspection of the weld between the lower core forging and the core barrel. Should maintenance be required on elements of the lower internal assembly below the lower core forging, it would be necessary to construct a tool capable of being submerged thirty feet and turned at a 90-degree angle in order to access the lower internal assembly. Maintenance on the lower face of the lower core forging would require the instrument to have an additional 90-degree turn to access the lower side of the lower core forging. Maintenance procedures using such a tool would be time consuming and would expose workers to radiation throughout the period in which the tool was used. Such a high level exposure to radiation is not desirable.
Therefore, there is a need for a method and apparatus to allow workers to repair the lower internal assembly of a nuclear reactor core assembly which would reduce the amount of exposure to radiation.